The 41-year-old was shortlisted for the accolade for the fourth time, having previously been chosen in 2002, 2005 and 2009.
She was finally awarded the honour by the Poetry Book Society on Monday, receiving a £15,000 prize.
Each of the shortlisted poets received £1,000.
Born in Portadown, Co Armagh, but currently living in Belfast, Morrissey is the author of five poetry collections.
She was appointed Belfast's in augural Poet Laureate in July.
Chair of judges Ian Duhig said: "In a year of brilliantly themed collections, the judges were unanimous in choosing Sinéad Morrissey's Parallax as the winner.
"Politically, historically and personally ambitious, expressed in beautifully turned language, her book is as many-angled and any-angled as its title suggests."
Belfast Lord Mayor, Máirtín Ó Muilleoir, has congratulated Morrissey, praising the poet for her "creative excellance".
Sinead is enriching the cultural aspect of our daily lives and building on our creative legacy.
Máirtín Ó Muilleoir, Belfast Lord Mayor
He said: "Our city is tremendously proud of her achievements.
"Through her work with communities across Belfast, this wonderful poet is engaging with residents and encouraging everyone to celebrate our poetry. On behalf of all of us, I send our congratulations."
Mr Ó Muilleoir created the post of Belfast Poet Laureate when he came into office last year.
Welcoming the news, the Arts Council's Head of Literature & Drama, Damian Smyth, said: "Sinéad has been for 15 years an outstanding voice in contemporary poetry in English and we are delighted that, once again, her work is being recognised at the highest levels.
"Four of her poetry collections have to date been shortlisted for the TS Eliot Prize - including Parallax - so it is extremely rewarding to see her hard work and literary brilliance recognised.
"The TS Eliot Prize has been won in recent years by many great Irish poets, including Seamus Heaney in 2006, so Sinéad is keeping good company.'
Culture Minister Carál Ní Chuilín said: "This is a prestigious award which acknowledges one of our leading artists.
"A vibrant and exciting writer, Sinéad has an infectious enthusiasm which is apparent in her work. Judges praised the winning collection, Parallax, as an ambitious exploration of differing perspectives in beautifully turned language."